STOCK BASED COMPENSATION (Details) - USD ($)	3 Months Ended				5 Months Ended	9 Months Ended		60 Months Ended
	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Mar. 31, 2014	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	May. 06, 2015	May. 05, 2010
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)		2,892,717		2,595,084		2,892,717	2,595,084
Options granted (in shares)						517,000	464,500
Options exercised (in shares)						(511,187)	(47,412)
Options canceled/forfeited (in shares)						(4,560)	(740)
Outstanding options at the end of the period (in shares)	2,893,970		3,011,432		2,893,970	2,893,970	3,011,432
Exercisable options at the end of the period (in shares)	1,197,470		1,351,312		1,197,470	1,197,470	1,351,312
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)		$ 26.65		$ 26.04		$ 26.65	$ 26.04
Options granted (in dollars per share)						50.09	42.90
Options exercised (in dollars per share)						18.27	17.95
Options canceled/forfeited (in dollars per share)						31.87	16.51
Outstanding options at the end of the period (in dollars per share)	$ 32.31		$ 28.77		$ 32.31	32.31	28.77
Exercisable options at the end of the period (in dollars per share)	$ 23.89		$ 22.29		$ 23.89	$ 23.89	$ 22.29
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding						5 years 3 months 15 days	5 years 3 months
Weighted-average remaining contractual term of exercisable options						4 years 4 days	3 years 11 months 27 days
Aggregate Intrinsic Value
Options exercised (in dollars)						$ 17,845,000	$ 1,250,000
Outstanding options at the end of the period (in dollars)	$ 61,522,000		$ 43,850,000		$ 61,522,000	61,522,000	43,850,000
Exercisable options at the end of the period (in dollars)	35,497,000		28,382,000		35,497,000	$ 35,497,000	$ 28,382,000
Weighted-average fair value of grants (in dollars per share)						$ 9.04	$ 7.76
Stock-based compensation expenses (in dollars)	1,100,000		900,000			$ 3,000,000	$ 2,900,000
Income tax benefit from stock-based compensation (in dollars)	400,000		$ 300,000			1,100,000	$ 1,000,000
Unrecognized stock-based compensation expense (in dollars)	$ 6,300,000				$ 6,300,000	$ 6,300,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)						$ 9.04	$ 7.76
Risk-free interest rates (as a percent)						1.53%	1.70%
Dividend yield (as a percent)						1.81%	1.94%
Expected volatility (as a percent)						22.89%	23.19%
Expected option life						5 years 2 months 16 days	5 years 2 months 1 day
Period for which annualized dividends is considered to calculate dividend yield						5 years
RLI Corp. Long-Term Incentive Plan (LTIP) and Omnibus Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period						5 years
Term of options						8 years
Age and period of service of the participant to be eligible for retirement						75 years
Omnibus Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for grant									3,000,000
Weighted Number of Options Outstanding
Options granted (in shares)									2,458,059
RLI Corp. Long-Term Incentive Plan (2010 LTIP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for grant								4,000,000
Weighted Number of Options Outstanding
Options granted (in shares)		53,500						2,878,000
RLI Corp. Long-Term Incentive Plan (2010 LTIP) | Outside director
Weighted average grant date assumptions and weighted average fair value
Restricted shares issued per recipient (in dollars)				$ 10,000
RLI Corp. Long-Term Incentive Plan (2015 LITP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for grant	4,000,000				4,000,000	4,000,000
Weighted Number of Options Outstanding
Options granted (in shares)					463,500
Outside Directors' Stock Option Plan (the director's plan)
Aggregate Intrinsic Value
Stock-based compensation expenses (in dollars)				$ 100,000
Weighted average grant date assumptions and weighted average fair value
Restriction period				1 year
Restricted shares issued				2,097